Taxonomize all uses of the fixed-size hash container types

[hdevalence]

The anti-design document describes the fixed-size hash container types, which over time became general-purpose byte containers, as a feature to remove from the codebase going forward.

To do this, we need to taxonomize all current uses of these types, and for each use, describe what type should be used instead. For instance, some uses of H32 represent a truncated double SHA256 checksum, and should be replaced by a new TruncatedDoubleSha256 (naming tbd) type.

[hdevalence]

Progress towards #63